1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method.
2. Description of the Related Art
There is widely known a method for generating test patterns outside a printing area on a photosensitive element or on a transfer belt and estimating density and position information from data for reflectivity of the test patterns so as to control image-forming process conditions for an image density and an image position or the like. For example, Document 1 (Japanese Patent Application Laid-open No. 2008-40441) discloses a configuration in which test patterns for controlling decision of image-forming process conditions are generated in a plurality of locations outside an image area and the image-forming process conditions are decided according to respective results of detection of the test patterns in the plurality of locations, which enables deviation of density to be hardly affected on the decision of the image-forming process conditions even if the deviation of density occurs caused by the locations. Thus, the disclosed configuration allows achievement of cost reduction and space saving of a cleaning device while adopting an intermediate transfer system with high accuracy of image superposition, and also allows reduction of downtime of the device due to process control for image formation using the test patterns and achievement of stable image quality.
Incidentally, recently, there have been developed color production printers for realizing color-on-demand printing for outputting a large number of color documents, at high speed, such as leaflets, catalogs, reports, and bills. This type of color production printer is for use in a case where, for example, tens of millions of telephone bills and receipts are issued within an issuance time limit of about one week. Thus, continuous printing is performed day and night during the period of one week (in other words, high-speed printing of hundreds of copies per minute is continuously performed for several tens of hours). From these situations, the high-speed type of color production printers is characterized in that the printer can never be stopped during continuous operation. This is because the stop of the printer operation may be caused to fail to meet the issuance time limit for the enormous number of copies. In this regard, the high-speed type of color production printers is greatly different in terms of technology from printers (multifunction peripheral (MFP)) installed in offices.
Meanwhile, the control of the image-forming process conditions disclosed in the Document 1 is performed on “offline control”, and thus the printing operation has to be stopped. Therefore, the control of the image-forming process conditions disclosed in the Document 1 cannot frequently be performed. Particularly, when the high-speed printing of hundreds of copies per minute is continuously performed for several tens of hours like the high-speed type of color production printer, the printing operation is stopped at a frequency of once in several minutes to perform the control of the image-forming process conditions, which is not advantageous to the characteristic of the high-speed type of color production printer that can never be stopped during the continuous operation. Moreover, if the continuous operation is performed without the control of the image-forming process conditions, the state of the process is largely changed, which causes degradation of image quality. More specifically, there is the necessity of a new configuration in which the control of the image-forming process conditions can be always implemented in real time on the high-speed type of color production printer without stopping the printing operation.
Disclosed, therefore, in Document 2 (Perry Y Li and Sohail A Dianat “Robust Stabilization of Tone Reproduction Curves for the Xerographic Printing Process” IEEE TRANSACTIONS ON CONTROL SYSTEMS TECHNOLOGY, VOL. 9, NO. 2, MARCH 2001) is a configuration method of a feedback control system for measuring a toner adhesion amount on the intermediate transfer belt or measuring an image fixed on a paper and adjusting and optimizing, in real time, set values of a charging device, an exposing device, and a developing device in an image forming engine using an electrophotographic process.
The charging device, the exposing device, and the developing device or the like in the image forming engine using the electrophotographic process interact with one another, and thus parameters for setting operations of these devices cannot be decided independently. Moreover, because an image to be measured has a plurality of colors or a plurality of brightness values/density values, it is necessary to perform “multiple-input and multiple-output control” for simultaneously deciding a plurality of manipulated variables. Here, the “manipulated variables” or “control inputs” in the control system are set values of the charging device, the exposing device, and the developing device, while “controlled variable” or “output” is a color or density/brightness to be measured on a sheet of paper with an image fixed thereon. Furthermore, in addition to the “multiple-input and multiple-output control”, an input-output relation in the electrophotographic process is generally complicated, and thus the same output cannot always be obtained with respect to the same input due to operating environments (temperature, humidity, etc.) and an operating time. As explained above, there is a problem that a model of the process also includes uncertain factors.
In order to solve the problems mentioned above, the Document 2 describes such a design method of a controller in which a difference between an output and a target value is caused to approach zero using a “robust control” method and stability is ensured under all cases of assumed uncertainties in operations of the process.
Incidentally, according to the Document 2, there is disclosed a feedback control system in which a relation between control input and gradation capability/color reproducibility of an output image is expressed as “linear model” and the robustness of the control system against non-linearity and uncertainty of the model is ensured. The method disclosed in the Document 2 allows frequent control of the process in real time without stopping the printing operation.
However, there are various constraint conditions for the set values (charging bias, exposing intensity, and developing bias, etc.) of the charging device, the exposing device, and the developing device in the image forming engine using the electrophotographic process. Each of the set values has upper limit/lower limit or strong limitation on a range of values which are caused to vary at a time. Moreover, the set values have mutual constraint. For example, a charging potential needs to fall within a certain range with respect to the developing bias. In addition, the relation among the set values of the charging device; the exposing device, and the developing device or the like; the color finally fixed on the paper; and the density/brightness of the color has high non-linearity.
As explained above, there is a big problem with the method disclosed in the Document 2 that the constraint conditions (upper limit and lower limit, etc.) for the “manipulated variables” or “control inputs” of the set values of the charging device, the exposing device, and the developing device in the image forming engine using the electrophotographic process cannot be considered.
Moreover, the relation between the control inputs and the gradation capability/the color reproducibility of the output image becomes nonlinear. An object being originally “nonlinear system” is approximated to be “linear system” and the “robust control” is applied to the object, and thus, there is also a problem that the object becomes a “conservative” control system with low transient response performance.
The present invention has been achieved to solve the conventional problems, and it is an object of the present invention to provide an image forming apparatus and an image forming method capable of improving the robustness related to the uncertainty of the process model related to image formation.